Combustion

Oxidising alkanes by combustion:

CH4(g)+2O2(g) → CO2(g)+2H2O(g) - Complete Combustion

Smaller molecules are more volatile so they burn more easily, but larger molecules are more exothermic as more bonds broken. Alkanes are good fuels; propane for central heating/cooking, butane for camping gas.

CH4(g)+1.5O2(g) → CO(g)+2H2O(g) - Incomplete Combustion

In a lack of oxygen carbon monoxide is formed instead of carbon dioxide. CO is extremely poisonous, they bind with the haemoglobin in the red blood cells better than oxygen. This will lead to oxygen deprivation thus headaches and nausea.

This reaction can be used to test the presence of double bonds. Adding orange bromine water will decolourise the solution. Mechanism: double bond repels the electrons in Br2, polarising it (δ+ and δ-). Heterolytic fission of Br occurs, one of them (Br+) attaching itself to the C atom. Br- then attaches to the carbocation formed

Adding hydrogen halides can cause 2 different products to be formed

4 of 14

Free Radical Substituition

Halogenoalkanes from Halogens and Alkanes

CH4 + Cl2 → CH3Cl + HCl

1. INITIATION - UV light forms Cl radicals - Cl2 → 2Cl・

2. PROPAGATION - Cl・+ CH4 → CH3・ + HCl

Repeated until no Cl2/CH4CH3・+ Cl2 → CH3Cl + Cl・

3. TERMINATION

e.g. CH3・+CH3・→ C2H6

Problems: Yield of CH3Cl isn't high as a mixture of products formed.

4. Further Substituition CH3CL → CH2Cl2

CH2Cl2 → CHCl3

CHCl3 → CCl4

5 of 14

Alkenes

Alkenes - Unsaturated hydrocarbons, has one or more double bonds CnH2n. Double bonds are reactive, they can open to join with other atoms. A double bond is made up of: σ - sigma bond - 2 s-orbitals overlap in a straight line - there's high electron density between them, which attracts electrophiles π - pi bond - 2 p-orbitals overlap. There are two parts to it - one above and one below the molecule, as the orbitalsare dumb-bellshaped.Double bonds cannot rotate due to this, causing cis/trans isomerism.

Addition polymerisation: double bonds in alkenes open up and join together to form long chain polymers ethene becomes poly(ethene)

Poly(chloroethene), also known as PVC, has a wide range of uses such as insulation on wires and water pipes

Poly(tetrafluoroethene), also known as PTFE, is useful on frying pans as they have non-stick properties

6 of 14

Alcohols

Alcohols

Hydroxyl Group -OH - Primary alcohols have 1 alkyl group attached to the OH carbon. Secondary alcohols have 2 alkyl groups, tertiary have 3. Small alcohols are miscible with water, as hydrogen bonds form between them (hydroxyl group of the alcohol is polar and so are the water molecules). Polar nature decreases as the size of molecule increases. Uses of ethanol: alcoholic drinks, methylated spirits (industrial solvent with purple dye, making it undrinkable), bioethanol

How they are made

1. Hydration of ethene (Hot Phosphoric Acid / 300℃ / 60atm

C2H4(g) + H2O(g) ⇔ C2H5OH(g)

Dehydration reverse reaction: phosphoric acid catalyst, 170℃

2. Fermentation of glucose (30-40℃ / Anaerobic Conditions / Yeist)

C6H12O6(aq) → 2C2H5OH(aq) + 2CO2(g) -

This is an exothermic reaction. Yiest dies when over 15% ethanol. Fractional distillation increases concentration of ethanol.

7 of 14

CFCs

Esterification (Conc Sulfuric Acid)

alcohol + carboxylic acid ⇔ ester + water

e.g. ethanol + ethanoic acid ⇔ ethylmethanoate + water

CFCs (chlorofluorocarbons)

Stable due to strong halogen-carbon bonds, volatile, non-flammable and non-toxic – CFCs found in aerosol cans, fridges, and air conditioning, until we found out that they were destructing the ozone layer. Alternatives: HCFCs (hydrochlorofluorocarbons), HFCs (hydrofluorocarbons) – these are temporary alternatives to CFCs. Less/no chlorine means less ozone depletion. They are eventually broken down in the atmosphere. However, these are much worse than CO2 molecules in terms of enhancing of the greenhouse effect. Nowadays aerosols have pump spray systems and ammonia is used in fridges, and CO2 is used to make foamed polymers instead of CFCs.

8 of 14

Oxidation of Alcohols

Oxidation of alcohol (Acidified Potassium Dichromate)

Combustion oxidises alcohols - they can be distilled/refluxed using an oxidising agent which turns from orange to green

PRIMARY ALCOHOLS distilled with oxidising agent producing ALDEHYDEs

ethanol + [O] → ethanal + water C2H5OH + [O] → CH3COH + H2O

To prevent further oxidation (forming carboxylic acid), removal of the aldehyde out of the oxidising solution is needed as soon as it is formed.

CARBOXYLIC ACID - REFLUX (continual heating) so that it is vigorously oxidised Vapourised compounds are condensed back into the reaction mixture with a vertical condenser.

ethanal + [O] → ethanoic acid CH3COH + [O] → CH3COOH

9 of 14

Infrared Spectroscopy

SECONDARY ALCOHOLS are oxidised to KETONES when refluxed with the oxidising agent.

propan-2-ol + [O] → propanone + water

CH3CH(OH)CH3 + [O] → CH3C=OCH3 + H2O

Ketones cannot be oxidised further by refluxing. Also, TERTIARY ALCOHOLS are not easily oxidised with oxidising agents (the only way is to burn).

Infrared spectroscopy – identifying molecules - Infrared radiation is passed through chemical sample. Different covalent bonds absorb different frequencies of this radiation. The graph shows ‘peaks’ at specific frequencies to show which bond is present.

Uses: breathalysing a driver will show the amount of ethanol vapour in their breath. CO absorbs a certain frequency of infrared radiation so it can be passed through sample air to identify CO gas.

10 of 14

Hydrolysis of Halogenoalkanes

Hydrolysis of halogenoalkanes - Nucleophilic Substitution

Halogens are more electronegative than carbon, thus carbon-halogen is polar. This means that C (δ+) easily attracts the nucleophiles (electron pair donator) e.g. OH- Bromomethane can be hydrolysed to ethanol:

C2H5Br + OH- → C2H5OH + Br-

Bond enthalpy decreases down the halogen group so iodoalkanes are hydrolysed the fastest (weaker bonds can be broken more easily).

OH- comes from warm aqueous potassium hydroxide (KOH) or sodium hydroxide (NaOH) and this is done under reflux.

Mechanism: OH- is attracted to the δ+ in C of bromomethane. Heterolytic fission of C-Br – electron pair is taken by Br. OH- attaches to the carbocation as Br- falls off.

Mass spectrometry - Uses: Probes to Mars have carried mass spectrometers to identify molecules on the planet. Level of pollutants in the air can be studied.

Mass spectrum gives mass/charge on the x-axis and the abundance on the y-axis. Molecules are bombarded with electrons to produce + ions of different fragments, and the bar represents molecular mass of the fragment.

Useful in differentiating between molecules that have the same molecular formula such as propanone and propanal, however propanal will have a C2H5+ fragment.